The present invention relates to fluid diffusion layers for fuel cells and in particular to gas diffusion layers for solid polymer electrolyte fuel cells. Further, it relates to the use of carbonized polymers containing pyrrolidone groups in the manufacture of fluid diffusion layers.
Fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Preferred fuel cell types include solid polymer electrolyte fuel cells that comprise a solid polymer electrolyte and operate at relatively low temperatures.
During normal operation of a solid polymer electrolyte fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The catalysts are preferably located at the interfaces between each electrode and the adjacent electrolyte.
A broad range of fluid reactants can be used in solid polymer electrolyte fuel cells and may be supplied in either gaseous or liquid form. For example, the oxidant stream may be substantially pure oxygen gas or a dilute oxygen stream such as air. The fuel may be, for example, substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or an aqueous liquid methanol mixture in a direct methanol fuel cell. Reactants are directed to the fuel cell electrodes and are distributed to catalyst therein through fluid diffusion layers. In the case of gaseous reactants, these layers are referred to as gas diffusion layers.
Solid polymer electrolyte fuel cells employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Each electrode comprises an appropriate catalyst, preferably located next to the solid polymer electrolyte. The catalyst may, for example, be a metal black, an alloy or a supported metal catalyst such as platinum on carbon. The catalyst may be disposed in a catalyst layer, and a catalyst layer typically contains ionomer, which may be similar to that used for the solid polymer electrolyte (for example, Nafion(copyright)). The catalyst layer may also contain a binder, such as polytetrafluoroethylene. The electrode may also contain a substrate (typically a porous, electrically conductive sheet material) that may be employed for purposes of mechanical support and/or reactant distribution, thus serving as a fluid diffusion layer.
The MEA is typically disposed between two plates to form a fuel cell assembly. The plates act as current collectors and provide support for the adjacent electrodes. The fuel cell assembly is typically compressed to ensure good electrical contact between the plates and the electrodes, in addition to good sealing between fuel cell components. A plurality of fuel cell assemblies may be combined in series or in parallel to form a fuel cell stack. In a fuel cell stack, a plate may be shared between two adjacent fuel cell assemblies, in which case the plate also serves as a separator to fluidly isolate the fluid streams of the two adjacent fuel cell assemblies.
Flow fields are employed for the purpose of directing reactants across the surfaces of the fluid diffusion electrodes or electrode substrates. Flow fields are disposed on each side of the MEA and comprise fluid distribution channels. The channels provide passages for the distribution of reactants to the electrode surfaces and also for the removal of reaction products and depleted reactant streams. The flow fields may be incorporated in the current collector/support plates on either side of the MEA (in which case the plates are known as flow field plates) or, alternatively, may be integrated into the fluid distribution layers of the electrodes.
The fluid distribution layers in such fuel cells may therefore have several functions, typically including: to provide access of the fluid reactants to the catalyst, to provide a pathway for removal of fluid reaction products, to serve as an electronic conductor between the catalyst layer and an adjacent flow field plate, to serve as a thermal conductor between the catalyst layer and an adjacent flow field plate, to provide mechanical support for the catalyst layer, and to provide mechanical support and dimensional stability for the ion-exchange membrane.
Preferably, the fluid distribution layers are thin, lightweight, inexpensive, and readily prepared using mass production techniques (for example, reel-to-reel processing techniques). Materials which have been employed in fluid distribution layers for solid polymer electrolyte fuel cells include commercially available carbon fiber paper and woven and/or non-woven carbon fabrics. However, the mechanical and/or electrical properties of these materials alone may not be adequate to meet all the requirements for fuel cell applications.
Consequently, appropriate fillers and/or coatings have been employed in the art to improve one or more of these properties. For instance, the electrical conductivity of a carbonaceous web might be increased by filling with an electrically conductive filler such as graphite particles plus a binder. (Carbonaceous in this context simply means containing carbon.) Alternatively, the stiffness of a carbonaceous web might be increased by impregnating the web with a suitable amount of curable polymer and then curing the polymer. Further, both stiffness and conductivity might be increased by impregnating the web with a carbonizable polymer, followed by carbonization of the polymer-impregnated web in an inert atmosphere. xe2x80x9cCarbonizationxe2x80x9d is defined herein as increasing the proportion of carbon by heating to temperatures of 600xc2x0 C. or greater in a nonoxidizing environment. During carbonization, carbon proportion increases as hydrogen, oxygen and nitrogen are evolved. The carbonization product remaining after carbonization can provide both mechanical support and additional electrical pathways throughout the web. Carbonization product means the reaction product after carbonization. Specific examples of webs filled with a binder include the thin, highly porous, non-woven carbon fiber web products of Technical Fibre Products Ltd., which typically comprise non-woven carbon fiber webs bound with a styrene-acrylic binder. While offering many desirable features, such products may not be sufficiently stiff for handling purposes nor of sufficient electrical conductivity for desired fuel cell performance. The styrene-acrylic binder present in the web is neither conductive nor carbonizable. However, both stiffness and conductivity of such webs may be suitably improved by impregnating with a carbon particle filler and a phenol-formaldehyde resin. The resin is then cured and carbonized leaving behind a substantial amount of carbonization product and resulting in a stiffer, more conductive web.
Phenol-formaldehyde resins have been commonly used in many impregnation applications. The phenol-formaldehyde resin is typically diluted in a carrier solvent and then used as an impregnant for fluid diffusion layers. However, a disadvantage of phenol-formaldehyde resins is that the uncured resin and by-products during curing are toxic as are certain preferred carrier solvents. Thus appropriate precautions must be taken when using these materials. Water can be used as a carrier solvent to some extent. However, in situations where one wishes to also add a conductive filler to the fluid diffusion layer, the preferable method of application is by applying an ink to the web, where the ink comprises the conductive filler, the resin, and the carrier solvent. Such an ink usually requires a relatively large amount of carrier solvent. If water is used as the carrier solvent in an ink, the water is present in an amount large enough to cause the resin to precipitate out of the ink. Thus, when using a water carrier solvent for a phenol-formaldehyde resin, a two step impregnation process is generally employed. An ink comprising the conductive filler is used in a first impregnation step and then a water/resin ink is added in a second step.
U.S. Pat. No. 6,037,073 employs a phenolic resin in its preparation of a combination bipolar plate/diffuser component. The combination component is prepared by making and screening an aqueous slurry mixture of carbon fibers (such as chopped or milled carbon fibers of various lengths) and about 20 wt % to about 50 wt % phenolic resin powder binder to produce a wet monolithic, which is subsequently dried at less than 80xc2x0 C. The dried monolith is further densified and resin-cured via mechanical compression in shaped graphite molds at about 120 to about 160xc2x0 C., and carbonized at about 700 to about 1300xc2x0 C. in an inert environment. A hermetic region on one side of the fluid diffusion layer is then achieved via conventional masking and chemical vapor infiltration (CVI) techniques. The porous region defines at least portions of reactant channels and the hermetic region contains coolant channels and prevents transport of fuel or oxidant to the wrong electrode of the fuel cell. U.S. Pat. No. 6,037,073 is incorporated by reference herein in its entirety.
Other carbonizable polymers may be considered for use in preparing fluid diffusion layers in a like manner to phenol-formaldehyde resins. For instance, polyacrylonitrile (PAN) polymers may seem to be a suitable choice since carbonizing processes are commonly used to make carbon products, such as carbon fiber webs from PAN polymers. In these processes, PAN is oxidatively stabilized prior to carbonization in order to obtain a substantial yield from the carbonizing. However, the PAN residue is extremely brittle and thus is not preferred for fluid diffusion layer applications. Additionally, toxic organic solvents are generally needed as carrier solvents.
Polymers having relatively low yields following conventional carbonization would typically be considered unsuitable for purposes of preparing fluid diffusion layers in this way. Pyrrolidone polymers, for instance, have typically provided relatively low yields following conventional carbonization.
An improved fluid diffusion layer for a fuel cell electrode comprises a porous carbonaceous web impregnated with a carbonization product of at least one polymer having pyrrolidone functionality. An improved fluid diffusion layer may comprise a plurality of porous carbonaceous webs impregnated with and bound together by a carbonization product of at least one polymer having pyrrolidone functionality. Pyrrolidone functionality is defined and discussed below. In the present context, xe2x80x9cimpregnatedxe2x80x9d means contained within, and the impregnated fluid diffusion layer does not require that all pores or voids are completely filled; in fact, it is specifically contemplated herein that the present fluid diffusion layers are impregnated but may still have substantial porosity. For example, the fluid diffusion layer preferably is at least about 50% porous. In the present context, the web impregnated with a carbonization product will preferably be made by impregnating the web with a carbonizable polymer having pyrrolidone functionality, followed by carbonization.
It has been found that polymers having pyrrolidone functionality are preferred carbonizable impregnants in the preparation of fluid diffusion layers for fuel cell electrodes. Similar to phenol-formaldehyde resins, carbonizable impregnants having pyrrolidone functionality can significantly improve the mechanical and electrical properties of certain porous carbonaceous webs for fuel cell applications. However, carbonizable impregnants having pyrrolidone functionality offer advantages in that a sufficient amount can be impregnated using water as a carrier solvent in a single impregnation step. The yield of carbonization product obtained from the impregnant having pyrrolidone functionality may be significantly increased by oxidizing the impregnant having pyrrolidone functionality prior to carbonization. Further, neither of the impregnant having pyrrolidone functionality or the water carrier solvent pose toxicity problems during impregnation.
The present method includes making a fluid diffusion layer for a fuel cell electrode that includes impregnating a porous carbonaceous web with a polymer and carbonizing the polymer wherein the polymer has pyrrolidone functionality.
An improved fluid diffusion layer can comprise a porous carbonaceous web, a non-particulate carbon filler, and a carbonization product of at least one polymer having pyrrolidone functionality, wherein the carbonization product binds the non-particulate carbon filler to the web. An improved fluid diffusion layer can comprise at least one porous carbonaceous web impregnated with a carbonization product of at least one polymer having pyrrolidone functionality, and the fluid diffusion layer defines at least one fluid distribution channel.
The present method also includes making a fluid diffusion layer by (a) coating a porous carbonaceous web with a mixture comprising a carbonizable polymer having pyrrolidone functionality; (b) disposing a non-particulate carbon filler in the coating of carbonizable polymer; and (c) carbonizing the carbonizable polymer so that the carbonization product binds the non-particulate carbon filler to the web.
As yet another aspect, a method of making a fluid diffusion layer defining a flow field is provided. The method comprises the steps of (a) impregnating at least one porous carbonaceous web with a polymer having pyrrolidone functionality, (b) carbonizing the polymer having pyrrolidone functionality to form a carbonization product, thereby forming a fluid diffusion layer having first and second major surfaces; and (c) forming at least one fluid distribution channel on the first major surface of the fluid diffusion layer.
Polymers having pyrrolidone functionality are derived from monomers containing a pyrrolidone functional group. Such monomers are represented by the following general chemical formula: 
where X represents chemical groups suitable for polymerization, such as for example, alkenyl groups. For example, X is a vinyl group in N-vinyl-2-pyrrolidone monomer.
In principle, the web may be impregnated with a monomer having pyrrolidone functionality that is polymerized in the web thereafter. A preferred polymer is polyvinylpyrrolidone, although other polymers (or suitable monomers) with pyrrolidone functional groups may be employed.
Polymers having pyrrolidone functionality vary widely in structure and include homopolymers of N-vinyl-2-pyrrolidone, and copolymers formed by polymerizing two or more polymerizable monomers, at least one of which provides pyrrolidone functionality. The term xe2x80x9cpyrrolidone functionalityxe2x80x9d means the presence of one or more pendant pyrrolidone rings.
The polymer having pyrrolidone functionality can be a homopolymer of N-vinyl-2-pyrrolidone or a copolymer of N-vinyl-2-pyrrolidone with one or more ethylenically-unsaturated copolymerizable monomers. The pendant pyrrolidone rings of the polymer can be substituted or unsubstituted. For example, it may be advisable to employ pyrrolidone rings substituted with alkyl, alkenyl, or other groups.
In the method, the polymer may be dissolved in a solvent and then applied in solution to the carbonaceous web. For environmental and other reasons, a preferred solvent is water.
The impregnated polymer is preferably stabilized before carbonizing. The stabilization step may increase the carbon yield after carbonization. This stabilization may be accomplished by heating the polymer in an oxidizing atmosphere at a temperature below about 420xc2x0 C. before carbonizing. Then, the carbonizing may be performed in an inert atmosphere at a temperature above about 600xc2x0 C.
The method is suitable for preparing fluid diffusion layers for various fuel cell embodiments operating either on gaseous or liquid reactants. However, the method is particularly suitable for preparing gas diffusion layers for use in solid polymer electrolyte fuel cell electrodes.
In the preparation of a fuel cell electrode from the fluid diffusion layer, an appropriate catalyst may be applied to the fluid diffusion layer either before or after the carbonizing step. Further, catalyst may be applied in a mixture which also comprises the carbonizable polymer having pyrrolidone functionality.
The carbonaceous webs employed in the present method are preferably non-woven carbon fiber mats comprising carbon fibers and a binder. The binder may be any suitable material including styrene-acrylic or even the carbonizable polymer having pyrrolidone functionality.
The method is particularly suitable for preparing acceptable fuel cell fluid diffusion layers using lightweight carbonaceous webs that are greater than about 80% porous, less than about 250 micrometers thick, and have a weight per unit area less than about 100 g/m2 (although materials with higher weights per unit area may also be suitable).
It may be advantageous to effect additional improvements (for example, improved electrical conductivity) by additionally incorporating carbon particles or non-particulate carbon filler (such as chopped carbon fibers) into the impregnated web. Suitable carbon particles include graphite particles, and they may be incorporated in a fill mixture comprising carbon particles and the carbonizable polymer having pyrrolidone functionality. Alternatively, it may be advantageous to incorporate non-particulate carbon filler as part of the fluid diffusion layer. Such a filler need not xe2x80x9cfillxe2x80x9d the porous carbonaceous web but may be disposed on a surface of such a web. The non-particulate carbon filler can be chopped carbon fibers. A fill mixture may comprise non-particulate carbon filler and/or carbon particles and a carbonizable polymer having pyrrolidone functionality. The fill mixture may also comprise a suitable pore former, such as methyl cellulose.
Through such methods, lightweight carbonaceous webs may be stiffened such that their Taber stiffness is greater than about 2 Taber units in the processing direction, or machine direction, of the porous carbonaceous web. Further, the electrical conductivity of such webs may be increased to be greater than about 1 (ohm-cm) xe2x88x921. The Gurley air permeability of the web however may be kept below about 20 seconds.
The present methods are suitable for preparing fluid diffusion layers for various fuel cell embodiments operating either on gaseous or liquid reactants. However, the methods are particularly suitable for preparing gas diffusion layers for use in solid polymer electrolyte fuel cell electrodes.
A method is also provided for preparing a fluid diffusion layer that defines one or more fluid distribution channels. For instance, a fluid diffusion layer can be made in accordance with the present techniques to be suitably thick to facilitate the formation of fluid distribution channels by embossing or other suitable means.